A conventional electronic computer system is equipped with a plurality of magnetic disk devices as external storage units for storing large amounts of data. In such a system, it is important to transfer large amounts of data for video processing among the individual disk devices at high speeds. However, the data transfer speed of the individual magnetic disk devices is limited by the speed of revolution of the magnetic disk and the like, and is difficult to increase.
Previous systems proposed a technique of transferring data simultaneously to the individual disk devices by synchronously driving the spindle motors that drive the disks. According to this technique, it is important for the spindle motors of the individual disk devices to have the same rotational phase. For instance, Japanese Patent Laid-Open No. 160877/1986 discloses a technique which finds the difference between the rotational phase of a reference spindle motor and the rotational phase of a spindle motor that rotates following thereto. This system then transmits a compensation signal which corresponds to the rotational phase difference of these motors to the revolution control circuit of the spindle motor of the follow-up side to drive each motor in synchronism.
According to the above prior art, each magnetic disk device has a reference revolution control circuit which includes a reference signal generating circuit such as a quartz oscillator that maintains the speed of revolution constant. When the spindle motors of the reference side and the follow-up side have an equal speed of revolution, the synchronous operation can be carried out sufficiently. However, the inventors have found the problem that it is difficult to drive the individual spindle motors at perfectly the same speed and the problem that when there is a difference in the speed of revolution of these spindle motors at the start of synchronous operation, a steady phase difference occurs depending upon the speed of revolution, making it impossible to achieve perfect synchronous operation.
When data is transferred in parallel from the individual magnetic disks to a higher-rank apparatus such as a host computer, it becomes necessary to temporarily store the data in a buffer memory of the data transfer circuit to absorb the difference of arrival time of the transferred data in order to bring the transfer of data into synchronism. The inventors, however, have found the fact that when the plurality of spindle motors have large difference in the revolution, their speed of buffer memory must have an increased capacity.